Uvarovite from Chromite-Bearing Ultramafic Intrusives, Orissa, India, a Crystal Chemical Characterization Using 57Fe Mössbau

American Mineralogist, Volume 95, pages 839–843, 2010

Uvarovite from chromite-bearing ultramafic intrusives, Orissa, India, a crystal-chemical characterization using 57Fe Mössbauer spectroscopy

Ta p a n Pa l 1,* a n d Di p a n k a r Da s 2

1Petrology Division, ER, Geological Survey of India, Kolkata 700 091, India 2UGC-DAE Consortium for Scientific Research, Kolkata 700 098, India

Ab s t r a c t EPMA analyses of uvarovite from the chromite-bearing ultramafic intrusive of Sukinda, India,

show that uvarovite containing 9.83 to 17.68 wt% Cr2O3 represents a solid solution of the binary

uvarovite-andradite series with Uva34–58 And42–58 . Associated chromite grains with 57–59 wt% Cr2O3 do not show any compositional variation from core to rim. 57Fe Mössbauer spectroscopic studies on the uvarovite samples undertaken both at room temperature (298 K) and low temperature (110 K) indicate the presence of Fe3+ only in octahedral coordination. Uvarovite grew from chromite (Cr source) and pyroxenes (Al, Fe, and Ca source) of the ultramafic host rocks. Keywords: Natural uvarovite, chromite, ultramafic intrusive, Eastern India

In t r o d u c t i o n some species contain small amounts of OH (Rossman and Aines The Cr-garnet (uvarovite) belongs to the ugrandite series 1991; Matsyuk et al. 1998). Naturally occurring uvarovite, al- 2+ 3+ though part of the ugrandite series, does not contain water, unlike with a general formula X3 Y2 (ZO4)3, where X = Ca and Y = (Al, Fe3+, and Cr3+) and Z = Si. Uvarovite, unlike other members hydrogrossular and hydroandradite. The reason for anhydrous of the ugrandite series, has very restricted occurrences (Deer et nature of Cr-garnet is not understood. al. 1992). The natural uvarovite always occurs as solid-solution The Sukinda (Orissa) ultramafic belt, situated on the southern series containing significant components of grossular or andra- fringe of the Singhbhum Craton, hosts the largest chromite de- dite (e.g., Proenza et al. 1999). Although readily synthesized posit in India. Chromite ores occur in two main zones: the brown experimentally (e.g., Deer et al. 1992; Proenza et al. 1999; ore zone and the gray ore zone (Mitra 1976). In the gray ore zone, Klemme et al. 2005), pure uvarovite end-member has yet to be the massive chromitite bodies are hosted in altered ultramafic found in nature. rocks in the form of serpentinized dunite and harzburgite. In Uvarovite has a broad field of stability, occurring in diverse the brown ore zone, chromite ores occur with serpentinite, talc- tremolite-chlorite schist, and a monomineralic clinochlore rock. geological environments such as: chromitites associated with Parrot-green uvarovite occurs only in the gray ore zone along basic and ultrabasic rocks (Jan et al. 1984; Graham et al. 1996; Proenza et al. 1999), rodingites (Mogessie and Rammlmair with pink Cr-chlorite (kammererite). Uvarovite grains are found 1994), skarns (von Knorring et al. 1986; Pan and Fleet 1989), within chromitite bodies either as minute discrete grains filling and kimberlites (Sobolev et al. 1973). Uvarovites other than fractures or as rims around chromite grains. skarns environment contain large amount of andradite or gros- Cr-garnet occurs commonly in uvarovite-grossular series sular components (e.g., von Knorring et al. 1986; Proenza et al. rather than in uvarovite-andradite series. (Deer et al. 1992; Proenza et al. 1999). Chromian andradite has only been reported 1999). The experimental work of Huckenholz and Knittel (1976) shows that addition of the andradite component into the uvarovite from some area (Jan et al. 1984; Duke and Bonardi 1982). The structure lowers its thermal stability. The experiments, however, hydrothermally generated garnet of Sukinda intrusive has a high- 57 also show a complete solid-solution series of uvarovite-andradite Cr content compared to other fields. To our knowledge, no Fe below 1137 °C at variable pressure and uvarovite-grossular Mössbauer studies on Cr-garnet have been published so far. 57 below 855 °C at 1 atm pressure (Huckenholz and Knittel 1975, We have carried out a Fe Mössbauer study of natural 1976). Uvarovites are generally considered to form from Cr- uvarovite occurring in chromite-bearing ultramafic intrusives, spinels by metasomatic reaction, but the exact nature of their and characterized its composition using EPMA. The chromites formation is still unknown (Arai et al. 1999; Proenza et al. 1999; closely associated with uvarovites have also been analyzed by Klemme et al. 2005). EPMA. The present study helps to understand the crystal-chemi- Related to the ugrandite series, hydrogrossular and hydroan- cal behavior of Cr-bearing garnets. In addition, the origin of the dradite contain large amounts (10–15 wt%) of water, whereas studied uvarovite is also addressed. garnets of the pyralspite series are nominally anhydrous, though Mineralogy and chemical analysis Uvarovite occurring in veins within chromitite displays green * E-mail: [email protected] color with variable hues from parrot green to grass green. The

0003-004X/10/0506–839$05.00/DOI: 10.2138/am.2010.3328 839